EctoATPase is an integral protein of most cell plasma membranes. This enzyme markedly changes upon cell transformation, rapid cell growth, loss of contact inhibition, marked exo- and endocytosis. It is inhibited by phenothiazines and tricyclic antidepressants. It is stimulated by some lectins. An exoATPase with properties similar to ectoATPase has been purified and antibodies to exoATPase bind to actoATPase. A double assay technique will be used to measure both changes in the ectoenzyme activity and the number of enzymes, and to determine whether such changes result from membrane turnover or enzyme activation. The modulation of extracellular ATP by ectoATPase and the role of ATP in modulating membrane permeability will be studied. The use of exoenzyme to study ecotenzyme will be extended to ectoprotein kinase.